International Journal of Scientific & Engineering Research, Volume 3, Issue 4, April-2012 1
ISSN 2229-5518
Dropwise and filmwise condensation
Saurabh pandey
Abstract—.The paper reviews progress in dropwise condensation research from 1930 to the present. Particular attention is given to heat transfer measurements, theory, effect the presence of air in the condenser has on the heat flux and surface heat transfer coefficient.. This experiment would be used in any industry which is trying to increase the efficiency of heat transfer. Subsequently, more accurate measurements have shown good consistency and the theory of the dropwise and filmwise condensation have become better unders- tood.. The balance of evidence suggests that dropwise condensation is a more effective method of heat transfer than filmwise condensa- tion, and the presence of air insteam vapour significantly reduces the heat transfer..The paper contains the experimental steps invove in performning the experiment precautions and final result of the experiment. The exact reading and values of heat transfer coefficient in both type of condensation is still not fully understood.
Index Terms—Condenser, Dimension less numbers, Dropwise condensation,Filmwise condensation , Heat transfer coefficient , ,Water flow rate,Mass flow rate,Non condensing gases
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Proceedings Steam may be condense onto the surface in two distinct modes, known a “filmwise” & ” dropwise” For the same temperature difference between the steam & the sur-
face, dropwise condensation is much more effective then filmwise condensation & for this reason the former is desirable although in practical plants it rarely occurs for prolonged periods. In filmwise condensation a laminar film of vapour is created upon a surface. This film can then flow downwards, increasing in thickness as additional vapour is picked up along the way .In dropwise con- densation vapour droplets form at an acute angle to a surface
.These droplets then flow downwards ,accumulating static drop- lets below them along the way. The second objective of this is to investigate the difference in heat flux between the two forms of condensation for the same set of conditions. Third objective is to investigate what effect the presence of air in the condenser has on the heat flux and surface heat transfer coefficient. This experiment would be used in any industry which is trying to increase the efficiency of heat transfer. An example of this is any vapour pow- er cycle such as the Rankine cycle. By increasing the efficiency of the condenser, its operational pressure can be reduced and the overall efficiency of the cycle can be increased.
2.1 Dropwise condensation
By specially treating the condensing surface the contact angle can be changed & the surface become ‘non – wettable’ .As the stream condenses ,a large number of generally spherical beads cover the surface. As the condensation proceeds ,the bead become larger, coalesce, and then strike downwards over the surface. The mov- ing bead gathers all the static bead along its downward in its trail. The ‘bear’ surface offers very little resistance to the transfer of heat and very high heat fluxes therefore possible.Unfortunately, due to the nature of the material used in the construction of con- densing heat exchangers, filmwise condensation is normal
.(Although many bare metal surfaces are ‘non-wettable’ this not is
true of the oxide film which quickly covers the bare material).
Dropwise and Filmwise condensation
2.3 Filmwise condensation
Unless specially treated, most materials are wettable as con- densation occurs a film condensate spreads over the sur- face.The thickness of the film depends upon a numbers of factors, e.g. the rate of condensation ,the viscosity of the con- densate and whether the surface is horizontal or vertical, etc.Fresh vapour condenses on to the outside of the film & heat is transfered by conduction through the film to the met- al surface beneath. As the film thickness it flows downward
& drips from the low points leaving the film intact & at an equilibrium thickness.The film of liquid is barrier to transfer of the heat and its resistance accounts for most of the differ- ence between the effectiveness of filmwise and dropwise condensation.
2.4 Methods and Procedure
Fill up the 5 litre distilled water in main unite by opening the valve.After filling the water close the valve. Start water flow
through one of the condensers which is to be tested and note
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International Journal of Scientific & Engineering Research Volume 3, Issue 4, April-2012 2
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down water flow rate in rotameter.Ensure that during measure- ment, water is flowing only through the condenser under test and second valve is closed.
Connect supply socket to mains and switch ON the heater switch
1/u = 1/hi + 1/h o× D/DO kcal/hr-m2-
Same procedure can be repeated for other condenser.Except for same exceptional cases overall heat transfer coefficient for drop- wise condensation will be higher than that of filmwise condensa- tion.
Result may vary from theory in some degree due to unavoidable heat losses.
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Saurabh pandey is currently pursuing bachelor degree program in chemi- cal engineering in National Institute of Technology,Jalandhar, India, PH-
09872833557. E-mail: saurabh.sleeping@gmail.com
The water level should be up to ¾ th of container.Do not start the heat supply unless water is fillked in test tube unit.Operate gently the sector switch of temperature indicator to read various temperature.Slowly generation will be start in stream generation of the unite and the stream rises to the test section, gets condensed on the tubes and fall down in the cylindrical space. Record the the temperature of painted condenser ,plane condenser ,water inlet to condenser and water out let to condenser.Depending upon the condenser dropwise and filmwise condensation can be visualized,If the water flow rate is low the steam pressure will rise in cylindrical region and pressure gauge will read the pressure. If the water flow rate is matched then condensation will
occur at more or less atmospheric or upto 1 kg/cm2 pres-
sure.Observation like water flow temperature of painted condenser
,plane condenser ,water inlet to condenser and water out let to con- denser.Depending upon the condenser dropwise and filmwise conden- sation can be visualized rates, pressure and noted down in the observa- tion table at the end of each set.
Steam pressure kg/cm2 | 0.65 | 0.60 | 0.50 | |||
Water flow rate LPH | 156 | 54.6 | 14.1 | |||
Condenser under test | Filmwise | Filmwise | Dropwise | |||
Painted condenser outer surface | T1 | 79.3 | 91.00 | 90.60 | ||
Plane condenser outer surface | T2 | 78.3 | 88.00 | 94.3 | ||
Steam | T3 | 125 | 124.1 | 124.4 | ||
Water inlet to condenser | T4 | 30 | 28.3 | 28.6 | ||
Water outlet plane condenser | T5 | 32.7 | 32.2 | - | ||
Water outlet painted condenser | T6 | - | - | 33.4 | ||
Ambient | T7 | 32 | 32 | 32 |
Analysis Normally steam will not be pressurized. But if the pres- sure gauge reads some pressure then properties of steam should
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International Journal of Scientific & Engineering Research Volume 3, Issue 4, April-2012 3
ISSN 2229-5518
be taken at that pressure or otherwise atmospheric pressure will be taken.
3).(175×10-3)
= 0.010445 m2
We will first find the heat transfer coefficient inside the condenser under test.for this properties of water are taken at the bulk mean temperature of water e.g. (Twi + Two)/2 where Twi and Two are wa- ter inlet & outlet temperature.
Following properties are required:
ρ1 = density of water (kg/m3)
pr = Prandtl number
Now calculation for Reynold’number
Red = 4 m/(π×ρ1×µ1×D1)
Where
Di =Inner diameter of condenser. = 1.75 cms
If this vale of Red=2100 then flow will be turbulent in pipe. Now Nusselt number.
Nul = 0.023(Red)0.8.(Pr)0.4
And hi = Nul.k/L kcal/hr.m3 (w/m2 )
All temperatures are taken in
Steam temperature, T∞ = 124.1
Condenser surface temperature,
Copper condenser, Tcu = 88.4
Chromium temperature,Tcr = 91.4
Flow rate = 455ml/30sec
= 0.0546 m3/hr
Rate of steam condensed = 780 ml/hr
= 3.12kg/hr
At temperature = (Ts + Tsteam)/2
= (124.1+88.4)/2
= 106.25
Pressure gauge = 0.5 kg/cm2
Density of steam condensed at
Temperature (Tin+ Tout)/2 = 995.6 Kg/m3
Now parameters used,
Now calculate the heat transfer coefficient on the outer surface of the condenser (ho). For this properties of water taken at bulk mean temperature of condenser e.g.
(TS + Tw) /2
Where Ts = Temperature of steam
Tw = Temperature of condenser wall
Properties needed are
K2 = Thermal conductivity kcal/ hr-m 
(w/m- 
)
ρ = density of water (kg/m3)
µ= Viscosity of condensate kgf-sec/m2 (kg/m.s)
hfg = Heat of evaporation kcal/kg. (540 Kcal/kg)
ho = { (hfg×g×ρ22×k23)/((TS – Tw)×µ ×L)}O.25
Hfg = 533.3 kcal/kg
Ρ = 954.3 kg/m3
µ = 274.4×10-6 N-s/m3
Pr = I.75
K = 0.5860 kcal/h-m-
= 276.3×10-6 N-s/m3
Heat flux = U (T∞ - Ts ) Q = UA (T∞ - Ts)
Over all heat transfer coefficient = ms×hfg /(A×∆T)
U = 3.12×533.3/((0.010445×(124.1-88.4))
= 4462.2 Kcal/h-m-
For filmwise condensation
ho = 0.943 {(HFG×ρ ×g×k )/((T∞ - Ts)×µ×L)}
2 3 0.25
where g = acceleration due to gravity =9.8 m/sec2 =1.27 × 108 m/hr2
L = Length of condenser =160 mm
From this value overall heat transfer coefficient (U) can be calcu- lated in
AREA
Outer diameter of heat transfer surface, d = 19 mm Length of heat transfer surface,L = 175 mm Heat transfer area π.d.L = π.(19×10-
h=0.943{(553.3×(954.3)2×9.81×(0.586)3)/((124.1-
88.40)×276.3×106×0.175)}0.25
h0 = 6304.209 kcal/h-m-
Red = 4×mw/(π×Di×ρ1×µ1) =v×ρ×D/ µ
mw = 54.3kg/hr volume flow rate
Q = 0.0546 m3/hr
velocity Q/A = 0.0546/(3600×0.000227)m/s
= 0.0668m/s
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Red>2100
Red = 0.0668×994.5×0.017/(8.01×10-6)
= 1410605
Nu = 0.023(Red)0.8×(Pr))0.4
Q = UA∆T
U =Q/A∆T
U = 4.27 ×533.5/(0.010445×(118.3-83.20)) U = 6213.65 Kcal/hr
Here, Pr = 5.204 at Tavg = (29 +34)/2 =31.5
Nu = 0.023(1410605)0.8×(5.204)0.4
= 3696.469 hi = Nu×k/L
= 3696.469×0.609/1.6
= 1360.76117
Tsteam =125
TS = 78.3
Tavg = (125+78.3)/2
=101.68
Pgauge = 0.6 kg/cm3
Density = 958.4 kg/m3
Hfg = 533.4 kcal/kg
K = 0.5883 kcal/h-m- Pr = 1.75
Ho = 0.943{(hfg×ρ2×g×k3 ) /((T∞ - TS )× ×L)}0.25
= 0.943{(533.4×(958.40)2×9.81×(0.58830)3/(125-78.30)×282.4×10-6)}
= 5885.32
Over all heat transfer coefficient
Q = ms×hfg
Q = 4.77 kg/hr
Q = UA(T∞ - TS)
U = ms×hfg/A(T∞ - TS )
= 4.77×5.33.4/(128-78.3)×0.010445
= 5216 kcal/h-m-
CALCULATION FOR DROPWISE CONDENSATION
Heat transfer area = 0.010445 m2
Tsteam = 118.3
Tchromum = 83.2
Tw in = 28.6
Tw out = 36.5
Pressure gauge 0.6kg/cm2
Mass of steam condensed = ms
Mass of coolded water = mw
Mass flow rate of cooling water = 220 kg/hr
Steam condensation = 4.27 kg/hr
Where U is heat transfer coefficient
The final observation is confirmed in the Handbook of Phase Change (2) which quotes that at atmospheric pressure, the Heat Flux in dropwise condensation can be more than filmwise. This can be explained in terms of how the condensation forms on the
condenser.
The vapour drops in dropwise condensation are discrete and are continually formed and released which means that the surface of the condenser is alsocontinually exposed. In comparison, the film created in filmwise condensation always covers the surface of the
condenser (3). As a relatively poor conductor of heat, this film creates a thermal resistance which is the reason why the value for Heat Flux is lower for filmwise in comparison to dropwise con- densation (3)
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To check the accuracy of the experiment, the values for the Heat Transfer Coefficient in the filmwise condenser were com- pared tothe values which are obtained theoretically using the Nusselt equation (3). Figure 2 shows that the results de- rived experimentally were of a lower value than of those de- rived theoretically.
One explanation for this is the presence of non-condensable gases in the steam vapour (1).It shows that for a certaintemperature difference, the Heat Flux for a condenser using steammixed with
5% of air is significantly smaller than pure steam, and the magni- tude of this difference increases with temperaturedifference. In the case of Heat Transfer Coefficients, the value for both steam and steam with air approaches zero, but when the steamis mixed with air it is consistently low.
5 ACKNOWLEDGMENT
With heart filled gratitude I would like to thank Prof. P.K.Mishra (IIT BHU), Prof. K.K.Singh (IIT BHU) for their untiring support and faith they placed in me during the tenure of my work. More- over, it is my duty to thank my Professors at NIT Jalandhar, namely Dr. A.K.Bansal (HOD Chemical Engineering), Prof. M.K.Jha and all others who encouraged me to persue this re- search and even take help from outside institutions for the same. Not to forget, I also intend on thanking the lab assistants at IIT BHU, who were always supporting and helping even at odd hours of my research. Finally, I would like to thank my par- ents and friends who had me motivated and spirited even when things were on a downfall.
6 REFERENCES
1.)Mayhew, Y, Rogers, G. (1992). Engineering thermodynam- ics:Work & Heat Transfer 4th ed. Prentice Hall.(2)
2.) Rose, J, Utaka, Y, Tanasawa, I. (1999).Handbook of Phase
Change: Dropwise Condensation. Taylor & Francis.(3)
3.)Incropera, F, DeWitt, D. (1996).Fundamentals of heat and mass transfer. 4thed. Wiley.
4.)CHEMICAL ENGINEERING’S HAND BOOK -ROBERT H. PERRY
5.).W. Rose, Dropwise Condensation Theory and Experiment: A Review, Proceedings of theInstitution of Mechanical Engineers, vol. 216, pp. 115–128, 2002.
6.) J.W. Rose, Condensation Heat Transfer Fundamentals, Transac- tions of the IChemE, vol.76(A), pp. 143–152, 199
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